1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory, and more particularly to a non-volatile semiconductor memory, such as an EPROM (Erasable and Programmable ROM), or EEPROM (referred to as Electrical Erasable and Programmable ROM, flash EEPROM, or flash memory), in which data are stored (written/read) by accumulating charges in a floating gate and detecting a change in a threshold voltage in terms of the presence or absence of the charges by a control gate.
2. Description of the Related Art
A technique of using a redundant memory cell built in a semiconductor memory in place of a memory cell which cannot be used because of its defect has been developed. The address of a defective memory cell can be known in the process of manufacturing the semiconductor memory. The address of the defective memory cell is stored previously in a redundant address. It is always observed whether or not the address generated from a normal address generating circuit is that of the defective cell. If so, the redundant memory cell is addressed in place of the defective memory cell. Namely, the address line having the defective memory cell is not used.
FIG. 3 is a circuit showing a part of an addressing circuit 1 of the semiconductor memory having such a redundancy function.
An address data for addressing is applied to the address generating circuit 1. An address data of a defective memory cell, which can be known from a test during the manufacturing process of the semiconductor memory cell, is applied to a redundant address generating circuit 2. A comparator 3 compares an output signal from the address generating circuit and that from the redundant address generating circuit 1. In this case, it generates a xe2x80x9cHxe2x80x9d level signal when the address data of the defective memory cell is generated from the address generating circuit 1, and an xe2x80x9cLxe2x80x9d level signal in other cases. A first switching means determines whether or not the output signal from the address generating circuit 1 should be selected according to the output from the comparator 3. Each of decoders decodes the address data from the first switching means 4. The decoding intends to designate the normal memory cell, or to designate the defective memory cell, i.e. to decode the address for redundancy. These decoders 5A, 5B, 5C and 5D are connected to word lines WL0, WL1, WL2 and WL3, respectively. Although not shown, a larger number of decoders and WL""s are actually connected.
When the normal address, but not redundant address is generated, the output signal from the address generating circuit 1 is applied to the decoders 5A, 5B, 5C and 5D through the first switching means 4. The output from the decoder corresponding to the selected WL becomes xe2x80x9cLxe2x80x9d level. Thus, the memory cell is designated.
When the redundant address is generated from the address generating circuit 1, the xe2x80x9cHxe2x80x9d level is generated from the comparator 3 so that the xe2x80x9cLxe2x80x9d level is applied to each of the AND gates constituting the first switching means 4. Thus, the address from the address generating circuit 1 is not applied to the decoders 5A, 5B, 5C and 5D.
On the other hand, the xe2x80x9cHxe2x80x9d level from the comparator 3 is applied to an AND gate 6 so as to be conductive. The addressing signal from the redundant address generating circuit 2 is sent to a redundant word line RWL through the AND gate 6 so that the redundant memory cell can be addressed.
Accordingly, in the device as shown in FIG. 3, the redundant address can be designated.
However, where the above semiconductor memory is a non-volatile semiconductor memory called a flash memory, as the case may be, the following problem happened.
Now, referring to FIG. 4, an explanation will be given of such a non-volatile semiconductor memory. Although the non-volatile semiconductor memory cell is roughly classified into a split-gate type and a stacked-gate type, the split-gate type non-volatile semiconductor memory as disclosed in WO92/18980 (G11C13/00) will be explained.
FIG. 4 is a circuit diagram of a memory cell array 21 in which non-volatile memories 20A, 20B, 20C and 20D are arranged in a matrix form. FIG. 5 is a view showing the structure of a certain memory cell therein.
As seen from FIG. 4, in adjacent non-volatile semiconductor memories 20A and 20B, and 20C and 20D, their source regions are commonly connected to a source line SL, and their drain regions are connected to bit lines BL0, and BL1, respectively. The control gates CG""s of the non-volatile memories 20A and 20C, and 20B and 20D are connected to word lines WL0 and WL1, respectively. In such a spirit gate structured non-volatile semiconductor memory, as shown in FIG. 5, since the control gate electrode CG is formed from on the upper surface of the floating gate electrode to on the side surface thereof, a distance between the control gate electrode CG and the bit lines becomes narrower. Therefore according to the progress of miniaturization of the device, the interval between the bit lines BL0, BL1 and control gates CG""s (word lines) becomes very narrow so that short-circuiting is likely to occur in comparison with the stacked gate structure in which control gate electrode is formed on the floating gate electrode. When the short-circuiting occurs, the memory at issue cannot be used and dealt with as a defective cell
An explanation will be given of the method of writing, reading or erasing data (charge) into or from the non-volatile semiconductor memory having such a structure. The following explanation will be particularly directed to the non-volatile semiconductor memory cell 20A.
The writing operation is made as follows. The voltages of e.g. 0 V, 11V and 2V are applied to the bit line BL0, source line SL and word line WL0, respectively. In this case, since a high voltage is applied to the source line, the potential of a floating gate FG which is strongly capacitive-coupled with a diffused layer (not shown) constituting the source line is raised to about 9 V. As a result, hot electrons generated between the drain region and source region jump into the floating gate FG, thus making the data writing operation.
The reading operation is made as follows. The voltages of e.g. 2 V, 0 V and 4 V are applied to the bit line BL0, source line SL and word line WL0, respectively. In this case, it is determined whether or not data. has been stored in the floating gate FG according to whether or not a reading operation current flows from the drain region to the source region. Specifically, when the reading operation current does not flow, the data has been stored in the floating gate. The erasure is made as follows. The voltages of e.g. 0 V, 0 V and 14 V are applied to the bit line BL0, source line SL and word line WL0, respectively. Then, the charge stored in the floating gate is pulled out toward the control gate CG so that the data is erased.
In the non-volatile semiconductor memory having the above structure, when all the word lines WL""s must be simultaneously selected for data erasure, if there is any leaking defective cell, the high voltage (14 V in the above explanation) required for erasure cannot be supplied to each control gate CG.
In FIG. 3, all the decoders 5A, 5B, 5C and 5D are selected for data erasure. The high voltage from the high voltage generating circuit 8 is applied to the word lines WL0, WL1, WL2 and WL3 through the decoders 5A, 5B, 5C and 5D, respectively. If there is leakage failure in any memory cell (cell directed to redundancy) connected to the word lines WL0, WL1, WL2 and WL3, the voltage on these word lines cannot be raised to about 14 V. Therefore, erasure cannot be normally made for all the memory cells connected to WL0, WL1, WL2 and WL3.
Now, the leakage failure refers to short-circuiting in the word line WL""s that is a phenomenon that excessive current flows through the word line WL. This is attributable to that poor connecting occurs between the word line WL and bit line BL.
Meanwhile, in some non-volatile semiconductor memories, the storage area is divided into a plurality of regions called sectors. One sector is a minimum unit for which collective erasure can be made. Using the sector facilitates local erasure.
On the other hand, there is a case where it is desired to make the erasure for the entire chip. To this end, the erasure can be made successively for each of the sectors. This lengthens the total time taken for erasure. Therefore, collective erasure is desired for plural sectors.
However, the entire chip may include the sector having the cell suffering from the above leakage failure (although the poor sector is replaced by the redundant sector, it is included for the object for erasure when collective erasure is made for the entire chip). If the chip includes such a sector, as described above, the high voltage required for erasure cannot be supplied to each control gate. Namely, the faulty sector which is an object for redundancy will affect the normal sector.
As a result, there was a serious circumstance which impedes the collective erasure in an essential meaning.
The present invention intends to provide a non-volatile semiconductor memory for which collective erasure can be made irrespectively of whether or not there is any faulty sector which is an object of redundancy.
A first aspect of the memory is a non-volatile semiconductor memory having a plurality of regions of sectors collectively erasable of stored data of the present invention, which comprises: a high voltage generating circuit for generating a high voltage used for erasing data for the non-volatile semiconductor memory; and a plurality of constant current circuits each connected between said high voltage generating circuit and said plurality of sector regions; wherein said plurality of constant current circuits are operated in collective erasure of the data so as to limit the current flowing through said plurality of regions of sectors.
A second aspect of the present invention is a non-volatile semiconductor memory according to the first aspect, wherein each of said plurality of constant current circuits is a current mirror circuit, and constant current transistors each of which produces an output current corresponding to an input current to said current mirror circuit are connected to said plurality of regions of sectors.
A third aspect of the present invention is a non-volatile semiconductor memory according to the first aspect, which further comprises a switch for releasing the constant current operation by said plurality of transistors during non-erasure operation of the data.
A fourth aspect of the present invention is a non-volatile semiconductor memory according to the first aspect, wherein said non-volatile semiconductor memory is a split-gate type of EEPROM, and each of said current mirror circuits is connected each of word lines of said EEPROM.
The present invention has been accomplished in order to solve the problem described above. As shown in FIG. 1, in accordance with the present invention there is provided a non-volatile semiconductor memory 20A, 20B, 20C, 20D (FIG. 4) having a plurality of regions of sectors 9A, 9B and 9C for which collective erasure of stored data can be made, comprising: a high voltage generating circuit 8 for generating a high voltage used for erasing data for the non-volatile semiconductor memory; a plurality of transistors 10A, 10B and 10C each connected between the high voltage generating circuit and the plurality of regions of sectors 9A, 9B and 9C; wherein constant current operation for the plurality of transistors 10A, 10B and 10C is performed in collective erasure of the data so as to limit the current flowing through the plurality of regions of sectors.
The present invention is characterized in that the non-volatile semiconductor memory 20A, 20B, 20C, 20D comprises a switch 19 for releasing the constant current operation by the plurality of transistors in non-erasure of the data as shown in FIG. 2.
In accordance with the present invention, in the collective erasure in the non-volatile semiconductor memory, since the high voltage required for erasure can be maintained even if there is any faulty sector among sectors, collective erasure can be made for all the sectors.
In accordance with the present invention, since the above effect can be obtained in such a manner that the transistor connected between the high voltage generating circuit and each sector is only operated in the saturated region. This does not lead an increase in the number of elements due to a complicated circuit structure.
Further, in accordance with the present invention, since the transistor connected between the high voltage generating circuit and each sector is caused to make the complete xe2x80x9cONxe2x80x9d operation during the non-erasure operation, during the other operation than the erasure operation, a sufficient current can be caused to flow, thereby suppressing reduction in a response speed.